1. Field of the Invention
The present invention relates to wireless communication and, more specifically, to a method and apparatus for performing a hybrid automatic repeat request (HARQ) in a multi-antenna system.
2. Related Art
Effective transmission/reception methods and utilizations have been proposed for a broadband wireless communication system to maximize efficiency of radio resources. An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference.
When a system uses the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner. In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain.
MIMO technology can be used to improve the efficiency of data transmission and reception using multiple transmission antennas and multiple reception antennas. MIMO technology may include a space frequency block code (SFBC), a space time block code (STBC), a cyclic delay diversity (CDD), a frequency switched transmit diversity (FSTD), a time switched transmit diversity (TSTD), a precoding vector switching (PVS), spatial multiplexing (SM) for implementing diversity. An MIMO channel matrix according to the number of reception antennas and the number of transmission antennas can be decomposed into a number of independent channels. Each of the independent channels is called a layer or stream. The number of layers is called a rank.
As disclosed in the section 6 of 3GPP (3rd generation partnership project) TS 36.211 V8.8.0 (2009 September) “Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation (Release 8)”, examples of downlink control channels used in 3GPP LTE include a physical control format indicator channel (PCFICH), a physical downlink control channel (PDCCH), a physical hybrid-ARQ indicator channel (PHICH), etc. The PCFICH transmitted in a 1st OFDM symbol of a subframe carries information regarding the number of OFDM symbols (i.e., a size of a control region) used for transmission of control channels in the subframe. The control information transmitted through the PDCCH is called downlink control information (DCI). The DCI indicates uplink or downlink scheduling information and an uplink transmit power control command for any user equipment (UE) groups. The PHICH carries an acknowledgement (ACK)/non-acknowledgement (NACK) signal for uplink hybrid automatic repeat request (HARQ). That is, the ACK/NACK signal for uplink data transmitted by the UE is transmitted through the PHICH.
A plurality of PHICHs can be allocated according to a system environment. In particular, the plurality of PHICHs needs to be allocated simultaneously in a carrier aggregation system for transmitting data by using a plurality of carriers, a MIMO system, etc. A base station (BS) allocates resources to the plurality of PHICHs, and transmits ACK/NACK through the PHICH.
There is a need for a resource allocation method for preventing PHICH resources from colliding with one another in a multi-user MIMO (MU-MIMO) environment.